So far, some digital camera products have been known to be capable of taking moving images. In recent years, interchangeable type digital cameras, especially an interchangeable lens compatible with the function of the camera body of taking moving images have also been under study.
Only the requirement for conventional cameras designed to take still images has been that they are in focus on target subjects in the moment of image shooting after determination of the composition, because their object has been not to miss blink-of-an-eye shutter chances. For instance, the so-called autofocus (AF) function has been adopted because of its combined high speed/accuracy.
In the taking of moving images with some video cameras designed for professional use, on the other hand, a skilled cameraman has implemented focusing operation in the manual focus (MF) mode. For many consumer-oriented video cameras, however, it is required to put the AF system constantly in operation thereby keeping an in-focus state depending on object distances. To this end, the contrast AF mode (the so-called climbing mode) relying upon an imaging device has been adopted.
Further, to keep the in-focus state, the focus lens is constantly subjected to back-and-fore, minute movement (called wobbling) in the optical axis direction to measure contrast changes. As the in-focus state is judged as being changing, it causes the focus lens to be properly moved for re-focusing operation. This wobbling function requires very fast operation depending on frame rates; so there is still mounting demand for reducing the weight of a lens used for wobbling.
The range of movement of the constantly moving focus lens remains within the depth of focus. Accordingly, it is controlled such that any out-of-focus state during wobbling cannot be recognized; however, when there is a large image magnification change, images look quite unnatural because they look as if they were always waving. Accordingly, it is still a vital requirement to reduce or minimize the magnification changes during wobbling.
Further, as there is a large change in the focus sensitivity (the amount of movement of the image plane per unit amount of movement of the lens) of the wobbling lens during zooming, it gives rise to a large difference in the amount of movement required for the wobbling lens at the wide-angle and telephoto ends, resulting in difficulty in control of fast and minute movement. It is therefore desired that the amount of change in the focus sensitivity of the wobbling lens be reduced as much as possible.
Patent Publication 1 has come up with a design example for small-format, less costly telephoto zoom lenses. According to the teachings, a positive-negative-positive three-groups zoom type is adopted and focusing is implemented at the second lens group, thereby making it possible to set up a small-format, less costly telephoto zoom lens with fewer lenses.
Patent Publication 2, and Patent Publication 3 has proposed a zoom lens finding an interchangeable lens system application for digital cameras with the taking of moving image in mind. With the zoom lens proposed in Patent Publication 2, focusing is implemented with fewer lenses.
For the negative power-leadoff type zoom lenses designed typically for size and cost reductions, on the other hand, such zoom lenses as set forth in Patent Publications 4, 5 and 6 have been known so far in the art. These are zoom lenses that are well fit for compact cameras and achieve a zoom ratio of the order of 3 with fewer lenses. They are operated in the rear focusing mode involving movement of one lens.
However, the zoom lens set forth in Patent Publication 1 is far away from being suitable for the taking of moving images because of too large image magnification changes during wobbling. In the zoom lenses set forth in Patent Publications 2 and 3, there is a fairly large number of lens groups used, which is apt to result not only in the need for using a lot more lenses, but also in bulkier size and higher costs.
The first aspect of the invention has been achieved with the above problems in mind, and has for its object to provide a negative-positive-negative-positive four-groups zoom lens favorable for small-format telephoto zoom lenses and having a screen easy to view during wobbling. Another object of the first aspect of the invention is to provide a zoom lens making it easy for an operator to view images during wobbling operation.
It is thus possible to provide a zoom lens that finds applications as an interchangeable lens system for digital cameras, is well compatible with the function of taking moving images, enables wobbling, and works more in favor of size and cost reductions. Yet another object of the first aspect of the invention is to provide an imaging apparatus equipped with such a zoom lens.
Referring again to the zoom lens disclosed in Patent Publication 2, its size is apt to grow large because it is a telephoto zoom lens of the positive power-leadoff type.
Referring again to the zoom lenses disclosed in Patent Publications 4 and 5, the taking of moving images is factored out, so intentional wobbling would result in large magnification changes and the inability to take moving images.
The second aspect of the invention has been figured out with the aforesaid problems in mind. One object of the second aspect of the invention is to provide a zoom lens working more in favor of size reductions and having a screen easier to view upon focusing, and another object is to provide a zoom lens easier to view images during wobbling operation.